ADA4511-2ARMZ-R7 datasheet(24/28 Pages) AD

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Data Sheet

ADA4511-2

APPLICATIONS INFORMATION

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Rev. A | 24 of 28

Figure 82 shows the TIA 1/β curve superimposed upon the open

loop gain of the amplifier. For the system to be stable, the 1/β curve

must have a slope of less than 20 dB/decade when it intersects

with the open loop response. In Figure 82 the dotted line shows

an uncompensated 1/β response (CF = 0 pF) intersecting with the

open loop gain at the frequency (fX) with a slope of 20 dB/decade

which indicates an unstable condition.

Figure 82. Generalized TIA 1/β and Transfer Function

The instability caused by CIN can be compensated by adding CF to

introduce a pole at a frequency equal to or lower than fX. The pole

frequency is as follows:

fP= 12πRFCF

(7)

Setting the pole at the fX frequency maximizes the signal bandwidth

with a 45° phase margin but is marginal for stability, as indicated

by the dashed line. Because fX is the geometric mean of fZ and the

gain bandwidth product frequency (fGBP) of the amplifier, calculate

fX by the following equation:

fX= fZfGBP

(8)

Substituting Equation 6 and Equation 7 into Equation 8, the CF

value that produces fX follows:

CF= 1+ 1+8πRFCINfGBP

4πRFfGBP

(9)

If 8π × RF × CIN × fGBP >> 1, Equation 9 simplifies to the following:

CF= CIN2πRFfGBP

(10)

Adding CF also sets the signal bandwidth at fP. Substitute Equation

10 into Equation 7 and rearrange the equation for the signal

bandwidth in terms of fGBP, RF, and CIN as follows:

fP= fGBP2πRFCIN

(11)

Note that the attainable signal bandwidth is a function of the time

constant RFCIN and the fGBP of the amplifier. To maximize the signal

bandwidth, choose an op amp with high bandwidth and low input

capacitance, and operate the photodiode in reverse bias to reduce

its junction capacitance.

Design Example

As a design example, Figure 83 shows one channel of the

ADA4511-2 configured as a TIA amplifier in a photodiode preamp

application. Assuming the photodiode has a CD of 5 pF, an ID of

2 µA, and the required full-scale VOUT is 100 mV, RF is 49.9 kΩ

according to Equation 2.

Figure 83. Single-Supply TIA Circuit Using the ADA4511-2

The ADA4511-2 input capacitance (CCM + CDM) is 22 pF, so the

total input capacitance (CIN) is 27 pF. By substituting CIN = 27 pF,

RF = 49.9 kΩ, and fGBP = 10 MHz into Equation 9 and Equation 11,

the resulting CF value and the −3 dB signal bandwidth (fP) are 3.1

pF and 1.1 MHz, respectively.

Figure 84 and Figure 85 show the compensations of the TIA circuit.

The system has a bandwidth of 1.1 MHz when it is maximized for a

signal bandwidth with CF = 3.1 pF. Increasing CF to 5.5 pF reduces

the bandwidth to 579 kHz. However, increasing the CF greatly

reduces the overshoot (see Figure 86). In practice, an optimum CF

value is determined experimentally by varying it slightly to optimize

the output pulse response.

Use the Analog Devices Analog Photodiode Wizard to design a

transimpedance amplifier circuit to interface with a photodiode.

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